1. Field of the Invention
The present invention relates to an apparatus for measuring the level of a liquid in a tank adopted in a liquid electrophotographic printer and, more particularly, to an apparatus for measuring the level of a liquid in a tank adopted in a liquid electrophotographic printer so that an error in measuring the level of a liquid due to solidification of developer can be reduced.
2. Description of the Related Art
An apparatus for measuring the level of a liquid contained in a tank is adopted in a liquid electrophotographic printer such as a laser printer or a copier using developer to provide information about the amount of developer remaining in the tank. The liquid electrophotographic printer informs a user to add developer at an appropriate time by displaying a message requesting refilling of the developer using information on the amount of remaining developer provided by the liquid level measuring apparatus. Thus, by requesting a supply of developer appropriately by means of the liquid level measuring apparatus, an incomplete print process or interruption of a print task during printing due to a lack of developer can be avoided.
FIG. 1 shows the structure of a general liquid electrophotographic printer. Referring to the drawing, a printer using developer includes a plurality of rollers 11, 12 and 13, a photoreceptor web 14, a reset unit 15, four laser scanning units 16, four development units 30, a drying unit 18, and a transfer unit 20.
The reset unit 15 includes an exposure unit 15a for emitting light to the photoreceptor web 14 to remove an electrostatic latent image, and a charging unit 15b for charging the photoreceptor web 14 to a predetermined electric potential. Reference numeral 39 denotes a waste developer collection tank.
The four laser scanning units 16 scan information of yellow (Y), magenta (M), cyan (C) and black (B) colors. The four development units 30 supply developer of yellow (Y), magenta (M), cyan (C) and black (B) colors.
Each of the development units 30 includes a developer supply tank 32 for supplying developer to the photoreceptor web 14 and a development tank 31 for collecting developer falling from the photoreceptor web 14. Each development tank 31 includes a development roller 36, a brush roller 37 for removing developer adhering to the development roller 36, a squeegee roller 34 for separating a liquid carrier component of the developer supplied to the photoreceptor web 14 which is not used for image formation, and a plate 35 for collecting the carrier component flowing along the outer circumferential surface of the squeegee roller 34 into the development tank 31.
The developer supply tank 32 is provided with a liquid carrier component, norpar (N), which is a solvent, and toner which is a development substance, or highly concentrated developer from a developer supply portion 38 and is provided with the developer from the development tank 31. The developer contained in the developer supply tank 32 is supplied between the development roller 36 and the photoreceptor web 14 by a pump P.
In the printing process of the liquid electrophotographic printer having the above structure, the laser scanning unit 16 scans light onto a predetermined area of the photoreceptor web 14 circulating past the reset unit 15. An electrostatic latent image is formed on the photoreceptor web 14 by the scanned light and the electrostatic latent image is developed by developer supplied from the developer supply tank 32 of the development unit 30. A color image is formed on the photoreceptor web 14 by the laser scanning units 16 for scanning light of different color information and the development units 30 for performing development with developer of a corresponding color. Most of the liquid carrier component that is not used for image development of the developer supplied to the photoreceptor web 14 from the development unit 30 during development is collected in the development tank 31. As the photoreceptor web 14 continues to rotate, the color image formed on the photoreceptor web 14 with toner which is a development substance is first transferred to a transfer roller 21 rotating in engagement with the photoreceptor web 14, after passing the drying unit 18 for absorbing and removing the liquid carrier component remaining on the photoreceptor web 14. Subsequently, the image on the transfer roller 21 is transferred to a sheet of paper 23 by the rotation of the transfer roller 21 and the fixation roller 22 which rotate in engagement with each other interposing the paper 23 inserted therebetween and make the paper 23 proceed.
There is a conventional apparatus for measuring the level of developer contained in the developer supply tank 32 by detecting the change in the amount of transmitted or reflected light with respect to the developer supply tank 32. The liquid level measuring apparatus includes a light source (not shown) for emitting light at the height corresponding to the level of liquid to be measured with respect to the developer supply tank 32, and a photodetector (not shown) for detecting light which is transmitted or the reflected light in a direction which is the same as or opposite to the light projecting direction with respect to the developer supply tank 32. The developer supply tank 32 is made of a material such as plastic in order to prevent easy breakage. However, most developer supply tanks made of plastic have a strong affinity to the developer. As a result, an error occurs in the measurement of the level of liquid due to developer adhering to an inner wall of the plastic developer supply tank without a change in the liquid level.
As another example of the liquid level measuring apparatus, a float (not 15 shown) installed in a tank and a sensor (not shown) for detecting the amount of the positional displacement of the float. For example, a permanent magnet is built-in the float and a Hall sensor responding to a magnetic force is adopted. In such a liquid level measuring apparatus, a movement path having a width slightly greater than the outer diameter of the float is vertically installed at the tank to minimize the rate of the positional displacement of the float in the horizontal direction with respect to a vertical movement path of the float. However, in this case, the width of the movement path of the float can be less than the outer diameter of the float due to the developer fixedly adhering to the tank so that an error can occur in measuring the level of liquid. Also, the float must be manufactured according to the specific gravity of the developer subject to the test, which is inconvenient.
Meanwhile, there is another method of measuring the level of liquid by installing two electrode plates (not shown) in a tank and detecting the change in electrostatic capacitance measured by the electrode plates corresponding to the change in the level of the developer. However, in this method, when the developer fixedly adheres to the electrode plate, an error can occur in measuring the level of liquid.
To solve the above problems, it is an objective of the present invention to provide an apparatus for measuring the level of liquid contained in a tank which can reduce fixing of developer to the position where the level of liquid is measured so that an error in measuring the level of liquid can be reduced.
Accordingly, to achieve the above objective, there is provided an apparatus for measuring the level of a liquid contained in a tank, which comprises a tank, at least one exhaust pipe including a transparent portion in at least a part thereof, having one end thereof installed to be connected to the tank and the other end, horizontally extended a predetermined length from the one end, connected to a set supply path and/or the tank so that liquid can be collected, said exhaust pipe being installed at a predetermined height vertically from a bottom surface of the tank to obtain information on the level of the liquid contained in the tank, a light source for emitting light to the transparent portion of the at least one exhaust pipe, a photodetector for detecting light reflected by or transmitted through the transparent portion of the at least one exhaust pipe and outputting electric signals corresponding to the detected light, and a liquid level measuring portion for measuring the level of the liquid contained in the tank from the electric signals output from the photodetector.
It is preferable in the present invention that the transparent portion is a glass pipe, and that the other end of the exhaust pipe is connected to a developer supply pipe through which the developer contained in the tank can be supplied to a photoreceptor medium.
Also, it is preferable in the present invention that, the at least one exhaust pipe comprises a first exhaust pipe. installed at a predetermined height from the bottom surface of the tank to provide information on a low level of liquid, a second exhaust pipe installed at a height higher than that of the first exhaust pipe to provide information on a medium level of liquid, and a third exhaust pipe installed at a height higher than that of the second exhaust pipe to provide information on a full level of liquid.